1. What is Spring Boot?

Answer:
Spring Boot is a Java-based framework that makes it easy to create stand-alone, production-grade Spring applications. It simplifies configuration by auto-configuring beans, using embedded servers like Tomcat, and reducing boilerplate code.

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2. What are the advantages of Spring Boot?

Answer:

• No XML configuration required
• Embedded servers (Tomcat, Jetty) — no WAR deployment needed
• Auto-configuration of Spring beans
• Production-ready with built-in monitoring and metrics
• Quick to set up and run (good for microservices)

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3. What is the difference between Spring and Spring Boot?

Spring (Core)	Spring Boot
Requires XML config	No XML — uses annotations & auto config
Need to deploy WAR	Uses embedded servers
Manual setup	Convention over configuration

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4. What is @SpringBootApplication?

Answer:
@SpringBootApplication is a combination of:

• @Configuration: marks class as a bean definition source
• @EnableAutoConfiguration: enables auto-config
• @ComponentScan: scans for components in the package

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5. How do you create a Spring Boot project?

Answer:

• Use https://start.spring.io
• Choose dependencies like Spring Web, Spring Data JPA, H2, etc.
• Download ZIP, unzip, open in IDE, and run main class.

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6. What is application.properties / application.yml?

Answer:
It’s a configuration file where you define app settings like port, DB URL, logging level, etc.

propertiesCopyEditserver.port=8081
spring.datasource.url=jdbc:mysql://localhost:3306/mydb

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7. What is a Spring Boot starter?

Answer:
A starter is a pre-packaged set of dependencies for common tasks. Example:

• spring-boot-starter-web → for REST APIs (Spring MVC + Tomcat + JSON)
• spring-boot-starter-data-jpa → for JPA/Hibernate integration

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8. How do you run a Spring Boot app?

Answer:

• Run main() method in the class annotated with @SpringBootApplication
• Or use mvn spring-boot:run or gradle bootRun

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9. What is an embedded server?

Answer:
It means the app runs its own web server (like Tomcat) internally — you don’t need to deploy WARs to an external server.

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10. What is dependency injection in Spring?

Answer:
It’s when Spring automatically provides the objects (beans) your class needs — instead of you creating them manually using new.

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11. What is @RestController?

Answer:
It’s used to create REST APIs. It’s a combo of:

• @Controller
• @ResponseBody (returns data directly, not a view)

javaCopyEdit@RestController
public class HelloController {
    @GetMapping("/hello")
    public String sayHello() {
        return "Hi there!";
    }
}

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12. What is the default port of Spring Boot?

Answer:
Port 8080. You can change it using:

propertiesCopyEditserver.port=9090

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13. How do you handle exceptions in Spring Boot?

Answer:
Using @ControllerAdvice and @ExceptionHandler.

javaCopyEdit@ControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(Exception.class)
    public ResponseEntity<String> handle(Exception e) {
        return new ResponseEntity<>(e.getMessage(), HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

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14. What is a Bean in Spring?

Answer:
A bean is just an object that’s managed by the Spring container — it’s created, injected, and wired by Spring automatically.

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15. What is @Autowired?

Answer:
Used to automatically inject a bean into another bean (dependency injection).

javaCopyEdit@Autowired
private MyService myService;

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16. What is @Component, @Service, @Repository, and @Controller?

Answer:

They all make classes into Spring-managed beans. The difference is semantic (used for readability and Spring’s internal behaviors):

• @Component – Generic stereotype for any Spring-managed class.
• @Service – Business logic layer.
• @Repository – Data access layer (adds exception translation for JPA).
• @Controller – Used in MVC to handle web requests.

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17. What is @GetMapping, @PostMapping, etc.?

Answer:

They are shortcuts for defining REST endpoints:

javaCopyEdit@GetMapping("/user")   // Handles GET requests
@PostMapping("/user")  // Handles POST requests
@PutMapping("/user")   // Handles PUT requests
@DeleteMapping("/user") // Handles DELETE requests

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18. How does Spring Boot handle database operations?

Answer:

With Spring Data JPA. You create an interface that extends JpaRepository, and BOOM — CRUD operations work out of the box.

javaCopyEditpublic interface UserRepository extends JpaRepository<User, Long> { }

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19. What is application.properties vs application.yml?

Answer:

Same purpose, different syntax:

• .properties uses key-value pairs
• .yml uses indentation-based YAML format

YAML is cleaner, especially for complex settings like DB config or nested properties.

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20. What is Spring Initializr?

Answer:

A web tool (start.spring.io) that helps generate a Spring Boot starter project with all dependencies, build tools (Maven/Gradle), Java version, etc.

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21. How do you connect Spring Boot with a database?

Answer:

By adding DB config in application.properties:

propertiesCopyEditspring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=pass
spring.jpa.hibernate.ddl-auto=update

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22. What is Spring Boot DevTools?

Answer:

A developer-friendly tool that enables:

• Auto-restart on code change
• LiveReload in browser
• Better logging

Add it via Maven:

xmlCopyEdit<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-devtools</artifactId>
</dependency>

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23. What is @RequestParam vs @PathVariable?

Answer:

• @RequestParam: gets query parameters
  /user?id=1
• @PathVariable: gets value from URI
  /user/1

javaCopyEdit@GetMapping("/user")
public String getById(@RequestParam int id) { }

@GetMapping("/user/{id}")
public String getById(@PathVariable int id) { }

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24. What is actuator in Spring Boot?

Answer:

Spring Boot Actuator provides endpoints to monitor and manage your app (health, metrics, info, etc.).

Add it with:

xmlCopyEdit<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

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25. How do you log in Spring Boot?

Answer:

Using SLF4J with Logback (default logger). Use:

javaCopyEditprivate static final Logger logger = LoggerFactory.getLogger(MyClass.class);

Control log level via application.properties:

propertiesCopyEditlogging.level.org.springframework=INFO
logging.level.com.myapp=DEBUG

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26. What is autoconfiguration in Spring Boot?

Answer:

Spring Boot automatically configures beans based on the dependencies on your classpath. Like, if you add spring-boot-starter-web, it sets up a dispatcher servlet, Jackson, etc., for you.

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27. What is a Microservice? Is Spring Boot good for it?

Answer:

A microservice is a small, independently deployable service that does one thing well. Spring Boot is perfect for microservices because it’s lightweight, fast to start, and has embedded servers.

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28. What is H2 database and why use it?

Answer:

H2 is an in-memory database (gone when app stops) — super useful for dev/testing. Add with:

xmlCopyEdit<dependency>
  <groupId>com.h2database</groupId>
  <artifactId>h2</artifactId>
</dependency>

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29. How do you create a REST API in Spring Boot?

Answer:
Just annotate your class with @RestController, and use mappings:

javaCopyEdit@RestController
public class UserController {

    @GetMapping("/users")
    public List<User> getAllUsers() {
        return userRepository.findAll();
    }
}

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30. What is the Spring Boot starter dependency for building web apps?

Answer:
spring-boot-starter-web → includes Spring MVC + Jackson (JSON) + embedded Tomcat.

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31. What is @Entity in Spring Boot?

Answer:

@Entity marks a class as a JPA entity — it maps to a database table.

javaCopyEdit@Entity
public class User {
   @Id
   private Long id;
   private String name;
}

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32. What is the role of @Id and @GeneratedValue?

Answer:

• @Id: Marks the primary key of the entity
• @GeneratedValue: Auto-generates the ID (like auto-increment)

javaCopyEdit@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

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33. What is Spring Data JPA?

Answer:

It’s a Spring project that simplifies database interactions using interfaces. You don’t write SQL — just extend JpaRepository.

---

34. What is the difference between CrudRepository and JpaRepository?

CrudRepository	JpaRepository
Basic CRUD operations	Includes pagination + batch ops
Lightweight	More features (e.g. flush)

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35. What is the use of @RequestBody?

Answer:

It maps the incoming JSON from a POST or PUT request to a Java object.

javaCopyEdit@PostMapping("/users")
public String createUser(@RequestBody User user) { }

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36. What is CORS and how do you enable it in Spring Boot?

Answer:

CORS = Cross-Origin Resource Sharing
To enable:

javaCopyEdit@CrossOrigin(origins = "*")
@GetMapping("/data")
public String getData() { }

Or globally via config.

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37. How do you return JSON in a Spring Boot API?

Answer:

Spring Boot auto-converts your Java object to JSON using Jackson:

javaCopyEdit@GetMapping("/user")
public User getUser() {
    return new User("Alice", 22);
}

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38. What is the default JSON library used in Spring Boot?

Answer:

Jackson (faster than GSON, widely supported)

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39. How do you validate input in Spring Boot?

Answer:

Use Bean Validation (javax.validation) + @Valid:

javaCopyEditpublic class User {
    @NotBlank
    private String name;
}

@PostMapping("/users")
public ResponseEntity<?> save(@Valid @RequestBody User user) { }

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40. How to handle 404 / 500 errors globally?

Answer:

Use @ControllerAdvice or customize ErrorController.

javaCopyEdit@ControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(Exception.class)
    public ResponseEntity<String> handleError(Exception e) {
        return new ResponseEntity<>("Something broke!", HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

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41. How do you use pagination in Spring Boot?

Answer:

Use Pageable with JpaRepository:

javaCopyEdit@GetMapping("/users")
public Page<User> getUsers(Pageable pageable) {
    return userRepository.findAll(pageable);
}

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42. What is the purpose of @EnableAutoConfiguration?

Answer:

It tells Spring Boot to guess and configure beans based on dependencies in the classpath. You rarely use it manually because it’s included in @SpringBootApplication.

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43. What is the use of profiles in Spring Boot?

Answer:

Spring profiles help manage different configs for dev, test, and prod environments.

propertiesCopyEdit# application-dev.properties
server.port=8081

Set profile via:

bashCopyEdit-Dspring.profiles.active=dev

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44. What is the use of @Configuration and @Bean?

Answer:

• @Configuration: Marks class for bean definitions.
• @Bean: Defines a bean to be managed by Spring.

javaCopyEdit@Configuration
public class AppConfig {
    @Bean
    public MyService service() {
        return new MyService();
    }
}

---

45. How to schedule tasks in Spring Boot?

Answer:

Use @Scheduled with @EnableScheduling:

javaCopyEdit@Scheduled(fixedRate = 5000)
public void printHello() {
    System.out.println("Hello every 5 sec!");
}

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46. How do you secure a Spring Boot app?

Answer:

Add Spring Security starter, configure auth using:

javaCopyEdit@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {
    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.authorizeRequests().anyRequest().authenticated().and().formLogin();
    }
}

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47. What are the common annotations used in Spring Boot?

Answer:

• @RestController
• @Autowired
• @RequestMapping
• @Service
• @Repository
• @Entity
• @SpringBootApplication

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48. How does Spring Boot reduce boilerplate code?

Answer:

• Auto-configuration
• Embedded server
• Starter dependencies
• Convention-over-configuration

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49. What is the structure of a typical Spring Boot project?

cssCopyEditsrc/
 └── main/
     ├── java/
     │    └── com.example.app/
     │         ├── controller/
     │         ├── service/
     │         ├── repository/
     │         └── Application.java
     └── resources/
          ├── application.properties
          └── static/

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50. What is Lombok and how does it help?

Answer:

Lombok is a Java library that reduces boilerplate code like getters/setters:

javaCopyEdit@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    private Long id;
    private String name;
}

Just add it as a dependency and let it auto-generate code.